Not only that, but human beings do not hear flat response as flat independent of volume. Try looking up Fletcher-Munson Curves. Our subjective experience of loudness changes with frequency and volume. This is why the loudness controly used to be so common- we need up to 20 dB of bass boost at low volume because at low frequencies what we hear drops off much faster at low volume than for midrange and treble. This is all on top of room response.
The best article on this I have seen was written by a recording engineer. They all know exactly what I'm talking about. In monitoring the recording the volume its played at affects the balance they hear which in turn affects the response they build into the recording. In short the only way to hear what they heard is to play it at the volume they heard it at. Which you have no way of knowing, yet perfectly explains why so many recordings have a playback volume that sounds "right".
Engineers know if they record what will sound flat at a loud volume that it will not sound right when played back either softer or louder than that. Its what they do. Now ask yourself how many times you've heard this fact discussed or even mentioned by the same audiophiles who think flat response is so important. I'm waiting. What's that? Never? Right.
Now where this really matters more than anywhere else is low bass. Subs. All these guys thinking EQ gonna solve all their problems. Every once in a while they will admit it only works at one location. Everywhere else is messed up even more than before. But never, ever do they mention the fact that EQ is volume dependent. What you need, if flat response is your goal, is EQ that varies with volume. Which no one has.
None of which is to say flat response is a hoax. Its a real thing. It can be measured. People can even listen and fairly reliably agree when response is tilted up or down or has peaks or dips. But there's a whole lot more to it than people think. And when they do every once in a while take the time to understand, well then guess what? They realize flat response is but one tiny little detail out of a whole encyclopedia of details, any one of which is just as important, or pretty darn close.